A Few Statistics Comparing SARS-CoV-2 with SARS-CoV and Influenza Pandemics

An interesting personal view by Professor Eskild Petersen and others in The Lancet published July 3 has some interesting characteristics when comparing the four different viral pandemics. 

The basic reproductive rate (R0) for SARS-CoV-2 is estimated to be 2.5, the range was 1.8 to 3.6 compared with 2 to 3 for SARS-CoV and the 1918 influenza pandemic, 0.9 for MERS-CoV, and 1.5 for the 2009 influenza pandemic. 

SARS-CoV-2 causes mild or asymptomatic disease in most cases.

However, severe to critical illness occurs in a small proportion of infected individuals, with the highest rate seen in people older than 70 years.

The measured case fatality rate varies between countries, probably because of differences in testing strategies.  Population-based mortality estimates vary widely across Europe, ranging from zero to high.  Numbers from the first affected regions in Italy, Lombardy, show an all age mortality rate of 154 per 100,000 population. 

This new virus, however, has a focal dissemination and therefore some areas have a higher disease burden and are affected more than others for reasons that are still not understood.  Nevertheless, early introduction of strict physical distancing and hygiene measures have proven effective in sharply reducing the R rate and associated mortality and could in part explain the geographical differences. 

Table 1 below shows clearly the differences between the four different viruses.  It shows that SARS-CoV-2 has the highest average R rate and therefore transmissibility.

Table 1: Characteristics of SARS-CoV-2, SARS-CoV, and pandemic influenza

SARS-CoV-2SARS-CoVPandemic influenza 1918Pandemic influenza 2009Interpretation
Transmissibility, R02·52·42·01·7SARS-CoV-2 has the highest average R0
Incubation period, days4–122–7Unknown2Longer incubation period; SARS-CoV epidemics form slower
Interval between symptom onset and maximum infectivity, days05–722SARS-CoV-2 is harder to contain than SARS-CoV
Proportion with mild illnessHighLowHighHighFacilitates undetected transmission
Proportion of patients requiring hospitalisationFew (20%)Most (>70%)FewFewConcern about capacity in the health sector
Proportion of patients requiring intensive care1/16 000Most (40%)Unknown1/104 000Concern about capacity in the health sector
Proportion of deaths in people younger than 65 years out of all deaths0·6–2·8%Unknown95%80%SARS-CoV-2 might cause as many deaths as the 1918 influenza pandemic, but fewer years of life lost and disability-adjusted life-years, as deaths are in the older population with underlying health conditions
Risk factors for severe illnessAge, comorbidityAge, comorbidityAge (<60 years)Age (<60 years)..

SARS-CoV-2 has a longer incubation period and this would therefore mean that epidemics form slower.  SARS-CoV-2 is harder to contain than SARS-CoV as the interval between maximum onset and maximum infectivity is zero days. 

SARS-CoV-2 has a high proportion of only mildly affected individuals with mild illness and therefore these individuals will remain undetected and able to transmit the virus.  Few patients, less than 20%, required hospitalisation and therefore concern about coping in the community is an issue. 

SARS-CoV-2 had 1:16,000 patients requiring intensive care as opposed to SARS-CoV, which required 40% of patients to have intensive care. 

SARS-CoV-2 had a proportion of deaths in people younger than 65 years out of all deaths of only 0.6 to 2.8% and as such SARS-CoV-2 might cause as many deaths as the 1918 influenza pandemic but fewer years of life lost and disability adjusted life years, as deaths are in the older population with underlying health conditions. 

Virus shedding appeared to be notably different between the SARS-CoV, SARS-CoV-2, and MERS-CoV.  SARS-CoV and MERS-CoV affected more the lower airways with less virus present in the upper respiratory tract whereas in SARS-CoV-2 the average viral load in a family cluster was 6.8 x 105 copies per upper respiratory tract swab during the first five days and a high viral load is present at the onset of symptoms but declined in the following 5 to 6 days. 

This quick decline in the viral load makes isolation and quarantine of patients with SARS-CoV-2 and their contacts much more challenging after illness onset in order to reduce transmission.  Whereas with SARS-CoV viral loads they peaked at 6 to 11 days after symptom onset, therefore allowing a full extra week to identify and isolate cases before transmission occurred.  This difference helps to explain why SARS could be eradicated in 2003 compared with the trajectory seen in the SARS-CoV-2 pandemic.

There is also increasing evidence of transmission from asymptomatic people, although what proportion of these individuals are pre-symptomatic remains unknown.  A study from Iceland found that 43% of PCR-positive cases had no symptoms, although some individuals showed symptoms later on.

Viral shedding might occur for prolonged periods.  A study of viral load in respiratory tract samples, faeces and blood from 96 patients with COVID-19 found a viral load of 105-106 copies/ml up to three weeks after symptom onset.  Viral shedding tended to be longer in stool samples; however, as of June 9, 2020, there is no documented evidence of any faecal-oral spread.

Viral load is higher and persists for longer in the lower respiratory tract of patients who are severely ill with COVID-19.  For SARS, lower respiratory tract infection occurred without upper respiratory tract infection.  As a consequence, transmission of SARS-CoV was infrequent during the illness and unlike transmission of influenza, transmission in household settings was very rare.

Case Fatality and Risk of Severe Illness

A key difference between SARS-CoV-2 and pandemic influenza was the age distribution of patients who are severely ill.  The mortality rate in people infected with SARS-CoV-2 increases steeply with age, and fatal outcomes are almost exclusively seen in people older than 50 years old.  This was also observed for SARS-CoV.

Because of the subset of patients who develop acute respiratory distress syndrome, there is a considerable demand for intensive care within patients who have COVID-19 even though most patients, 90%, have only mild clinical illness.  This requirement for respiratory support is higher for SARS-CoV-2 cases than within the influenza pandemic in 2009. 

A study in Yorkshire showed that only 14% required intensive care and a study from Denmark showed that only 4.5% of the total national intensive care unit bed capacity was used in the H1N1 pandemic. 

Population-Based Mortality

Interestingly enough, data showed high COVID-19 associated excess mortality in countries including Italy, Spain, the United Kingdom and Sweden whereas other countries such as Germany, Norway, and Greece have found no, or low, excess mortality, whereas the influenza pandemic in 1918 had an extreme excess mortality to a mild excess mortality in the 2009 influenza pandemic.

In the deadly 1918 influenza pandemic estimates show that about 1-2% of the global population died, whereas in the 2009 pandemic about 0.04% of the global population died. 

SARS-CoV-2 and Warmer Weather

Experimental data suggests that SARS-CoV-2 might be less able to survive in the summer. 

SARS-CoV-2 and the Effect of Containment Measures

The consensus is that rigorous mitigation measures are needed early to slow down SARS-CoV-2 transmission.

Historical evidence from influenza pandemics which occurred in the past century shows us that pandemics tend to come in waves over the first 2 to 5 years as the population immunity builds-up naturally or through vaccination and at that stage the number of infected cases tends to decrease. 

However, the near future will require a transition to a new normal, in which a combination of physical distancing, enhanced testing, quarantine, and contact tracing will be needed for a long time.

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